a) the FontLab VI Macro system to show the results of macro program interpretation from the Scripting panel;
b) the FontLab VI OT features compiler to show error messages and warnings that come up from the features written in the Features panel.

The Output panel, which looks like a basic text-output window with a scroll bar, opens automatically when there is something to show:

You can Ctrl-click on the panel to open the context menu:

To erase all of the panel’s contents, select Clear in menu or click on the Clear button.